1. Field of the Invention
The present invention relates to an image pick-up device for converting an optical signal into an electrical signal and a radiation imaging apparatus using such a device.
2. Discussion of the Background
An image pick-up device using a photoelectric conversion element is widely used for such applications as a video camera and a digital camera. Also, in an X-ray imaging apparatus constituting an example of a medical radiation diagnostic apparatus, the image pick-up device has recently replaced the silver salt film. In recent years, the trend in the medial field is toward more and more medical data on patients stored in a data base for the purpose of performing medical treatment rapidly and accurately. This is in view of the fact that a patient often consults a plurality of medical institutions and an accurate medical treatment would be impossible without referring to the data stored in other medical institutions. An example is the problem of drugs prescribed by different medical institutions and the side reactions between them. It is necessary for a given medical institution to perform proper treatment taking the drugs prescribed by other medical institutions into consideration.
A demand is also high for introducing a data base of radiographic image data, and digitization of radiographic images is desired. Conventional medical radiation diagnostic apparatuses have taken a radiographic image using a silver salt film. If this film is to be digitized, after being exposed and developed, it is required to be scanned by a scanner or the like. This consumes a considerable labor and time. Recent years have seen the realization of direct digitization of an image using a CCD camera of about one inch. In such an application as taking a picture of the lung of the human being, however, the need of covering an area of about 20 cm.times.30 cm requires an optical system for focusing light sufficient for this size, thereby leading to the problem that a larger apparatus is required.
A method for solving the problems of these two types of systems has been suggested using a thin-film transistor such as an a-Si TFT (amorphous silicon thin-film transistor) for the radiation imaging apparatus. FIG. 1 is a diagram showing a general configuration of an image pick-up device using the a-Si TFT. In the image pick-up device shown in FIG. 1, a pixel e1,1 at an address (1,1) is configured of an a-Si TFT 144, a photo-electric conversion film 140 and a pixel capacitor 142. Pixels e of this configuration are formed in an array (hereinafter referred to as the TFT array) of 2000 by 2000. The pixel at address (2000,2000), for example, is represented by e2000,2000. Each photoelectric conversion film 140 is impressed with a bias voltage from a power supply 148. The a-Si TFT 144 is connected to a signal line 118 and a scanning line 113 and the on-off operation thereof is controlled by a scanning line drive circuit 152. Each signal line 118 has an end thereof connected to a signal detection amplifier 154 through a change-over switch 146.
With the entrance of light into the pixel, a current flows in the photoelectric conversion film (photodiode) 140, and charge is accumulated in a photoelectric conversion circuit including the photoelectric conversion film and the pixel capacitor 142. The scanning line drive circuit 152 drives a scanning line 113. When all the TFTs connected to one scanning line 113 are turned on, the charge stored in the associated photoelectric conversion circuits is transferred to the amplifier 154 through the corresponding signal line 118. The change-over switch 146 is used to apply the charge for each pixel to the amplifier 154 thereby to convert it into a dot sequence signal that can be displayed on a display unit such as a liquid crystal display. The charge amount is dependent on the amount of the light entering the pixel, so that the output amplitude of the amplifier 154 undergoes variations.
In the system using the image pick-up device shown in FIG. 1, the output signal of the amplifier 154 is A/D converted thereby to produce a digital image directly.
If the image pick-up device using the TFT as mentioned above is to be applied for medical treatment, however, several problems are required to be solved. First, a higher image quality is required. Especially, the problem is how to reduce noises. Unless a clear image free of noises is obtained, an image pick-up device is useless for medical applications. The charge stored in the pixels is transferred to the amplifier 154, which generally has a high input resistance. The output of the amplifier 154, therefore, is affected even by a small disturbance, causing the deterioration of the image quality.
A cause of the disturbance is the noises derived from the scanning lines 113. Each signal line 118 forms a capacitance by crossing the scanning line 113 through an insulating layer. With the variations in source potential of the scanning line drive circuit 152, the capacitance at the crossing of the scanning line 113 and the signal line 118 is charged, and the resulting charge is also transferred to the amplifier 154. Although the description above refers to the case where a crossing capacitance is formed by crossing between the signal line 118 and the scanning line, the signal line 118 may cross wirings other than the scanning line to form a crossing capacitance.
FIG. 2 is a diagram schematically showing the crossing between the scanning line 113 and the signal line 118. FIG. 3 is a sectional view of the crossing between the scanning line 113 and the signal line 118 shown in FIG. 2. The scanning line 113 configured of a metal like MoTa or MoW, for example, is formed on a glass substrate 111, on which a gate insulating film 115 made of SiO.sub.X or SiO.sub.X is formed. The signal line 118 and the scanning line 113 are formed by being insulated from each other by the gate insulating film 115.
Assume that both the scanning line 113 and the signal line 118 have a width of 10 .mu.m (with the crossing area S of 100 .mu.m.sup.2), the gate insulating film has a thickness d of 0.3 .mu.m and has a dielectric constant .mu. of 5. The capacitance C at the crossing is given as EQU C=.epsilon.s/d=(5.times.8.854.times.10.sup.-12).times.(10.times.10.sup.-16) .sup.2 /(0.3.times.10.sup.-6)=14.8 fF (1)
In the case where there are 1000 scanning lines, each signal line 118 has a crossing capacitance of 14.8 pF. As a result, a 1-mV fluctuation of the source potential of the scanning line drive circuit would generate a charge of 14.8 fC in the amplifier 154. The charge stored in the pixel is substantially in the same order of magnitude (several tens of 10 fC to several pC). Therefore, the charge from each crossing between the scanning line 113 and the signal line 118 is a great cause of image quality deterioration.
Especially, a medical image pick-up device which requires a high dynamic range encounters the problem that the dynamic range must be sacrificed to accommodate the effect of the noises. Also, the conventional radiation image pick-up device poses the problem that an increased radiation dosage is required to produce a clear image. This gives rise to a serious problem of an increased exposure of patients to radioactivity.
An image pick-up device using the TFT array, on the other hand, has the problem of a read error and the leak current in addition to a deterioration of image quality described above. Specifically, in the image pick-up device using the TFT array described above, an image signal is produced by sequentially reading out the charge stored in each pixel, and the read characteristics vary minutely from one pixel position to another in the read operation. More specifically, in the case where the charge stored in the pixels is read out, the required read time (drive time) is determined by the on-resistance of the TFT used for switching and the time constant of the pixel capacitor. If the read time is short for the time constant, the charge stored in the pixel cannot be sufficiently read out, thereby causing an error (noise) of the stored charge and the read signal. This read error is one of the noises unique to the TFT array.
The leak current which is another noise unique to the TFT array is a phenomenon in which a minute current flows when the TFT is off. This noise stems from the insufficient off-resistance of the TFT. In such a case, the charge held in the pixel flows out so that the stored charge cannot be rightly read out, leading to an error (noise).
As described above, the two noises unique to the TFT array include the read error and the leak current. The large problem in eliminating these noises is the trade-off between the two types of errors. Specifically, the longer the read time, the smaller the read error. With the increase in the length of the read time (i.e., with the increase in the off-time), however, the error due to the leak current increases.
These errors are also affected by the ratio between the channel width and the channel length (W/L) of the TFT. In other words, a larger W/L ratio decreases the on/off resistance. The read error, therefore, decreases at the expense of a larger leak current.
Consequently, designing a TFT is crucial in a manner to minimize the two errors including the read error and the leak current.
The radiation diagnostic apparatus is used in two modes including radiography (still image) for taking a radiographic picture and fluorography (animation) for checking the condition of the affected part. The radiograhy taking one picture requires about one second of drive time, while the fluorography requires repetitive drives of about one-thirtieth of a second each. Therefore, the optimum TFT varies between the two modes. If the TFT is optimized only for one of the modes, therefore, the error is increased for the other mode to such an extent as to deteriorate the characteristics resulting in the loss of the practical value of the diagnostic apparatus.
For the reasons described above, an image pick-up device configured of the same TFT array has been difficult to use for both the radiography and the fluorography having different drive times.
In another conventional method, two types of arrays, one for radiography and the other for fluorography, are provided to switch between them according to the prevailing mode. This method, however, increases the system cost and contradicts the demand for a smaller size and a lighter weight.